Engineering Rock Mechanics

104 Intact rock

Time-dependent effects. We have indicated that during the complete
stress-strain curve, microcracking occurs from a very early stage in the pre-
peak region. For some purposes, it is convenient to assume that much of
the pre-peak portion represents elastic behaviour. However, there is no
time component in the theory of elasticity; yet, because of the continually
increasing microstructural damage even in the 'elastic' region, we would
expect some time-dependent behaviour.
There are four main time-dependent effects which are discussed here.

(a) strain rate-the total form of the complete stress-strain curve is a

@) creep-a material continues to strain when the applied stress is held

(c) relaxation-there is a decrease in stress within the material when the

(d) fatigue-there is an increase in strain due to cyclical changes in stress.

function of the applied strain rate;

constant;

applied strain is held constant;

These four effects are shown in Fig. 6.16 and are all manifestations of the

time-dependent nature of microcrack development.

The effect of a reduced strain rate is to reduce the overall elastic modulus

and the compressive strength. Creep from a point A in Fig. 6.16 is indicated

by the line AC. Relaxation is indicated by the line AR. Fatigue is indicated

by the stress cycles. The relation between these effects can be seen

especially from the form of the complete stress-strain curve at lower and

lower strain rates. Depending on whether the control variable is stress or

strain, the rock will be continually creeping or relaxing, respectively, during

generation of the complete stress-strain curve.

We have noted that stress cannot be used as the control variable to obtain

the post-peak region of the curve; nor indeed, as indicated by the line BC

in Fig. 6.16, can creep occur in the post-peak region without instantaneous

failure. As indicated by the lines AR and BR, relaxation can occur on either

side of the curve for a Class I curve. Also indicated in the figure are the lines

Constant

6

Tertiary

Secondary

Primary creep

6 t

F

s

Figure 6.16 Time-dependent effects and the complete stress-strain curve.